The present invention relates to magnetosensitive or galvanomagnetic devices (e.g. Hall generators, magnetoresistors, etc.) arranged in arrays for use as digital torque sensors.
Only analog sensors are available or known for use as torque sensors in automotive applications such as electronic power steering (EPS), engine control systems, etc. The majority of torque sensors use strain gages attached to a rotating shaft whose signal is transmitted through slip rings. In some cases, inductive coupling, infrared, or radio frequency methods are utilized for transmission instead of slip rings. These types of sensors arc highly accurate, but they are not suitable for in-vehicle applications due to the fragile nature of strain gages and the way they have to be attached to the shaft surface.
Presently, the torque sensor selected for EPS is of the resistive film type, which works in conjunction with a torsion bar. Essentially, it is a potentiometer translating the angle of the torsion bar twist into an electrical resistance value. It requires slip rings for signal transmission and, at least in principle, might have limited life due to localized film wear-out caused by extensive dithering.
Noncontacting compliant torque sensors utilizing a torsion bar to convert a twist of the torsion bar into torque by measuring the angular offset between the ends of the torsion bar are also known in the art. These torque sensors utilize various analog measurement techniques.
The use of magnetosensitive or galvanomagnetic devices, such as magnetoresistors (MRs) and Hall devices, as non-contacting position and angle sensors is well known in the art. For example, a magnetically biased differential magnetoresistive sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. Nos. 4,835,467, 5,731,702, and 5,754,042.
In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative, and in close proximity, to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.
Increasingly, more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. No. 5,570,016.
Single element magnetic field sensors composed of, for example, an indium antimonide or indium arsenide epitaxial film strip supported on, for example, a monocrystalline elemental semiconductor substrate, are also known. The indium antimonide or indium arsenide film is, for example, deposited either directly on the elemental semiconductor substrate or on an intermediate film that has a higher resistivity than that of silicon. A conductive contact is located at either end of the epitaxial film, and a plurality of metallic (gold) shorting bars are on, and regularly spaced along, the epitaxial film. U.S. Pat. Nos. 5,153,557, 5,184,106 and 5,491,461 exemplify examples thereof.
Many kinds of measurements require high accuracy and high resolution, and, as such, cannot easily be performed with common magnetic sensors comprising a single sensing element or dual sensing elements. Improved accuracy and resolution measurements can be achieved, however, using an array. The most common magnetic sensing element, the Hall effect element or device, does not quite fit the requirements for an array. Being a 4-terminal device complicates the array connections. Furthermore, its low output signal mandates the use of an integrated amplifier for each sensing element increasing the die size and its cost. Nonetheless, appropriately configured Hall sensors can be incorporated into Hall arrays.
However, compound semiconductor MRs, such as those manufactured from InSb, InAs, etc. are simply two-terminal resistors with a high magnetic sensitivity. Thus, compound semiconductor MRs are very suitable for the construction of single die MR array geometries for use as large range angular position sensors. In most cases, one terminal of all the MR elements can be common.
Ultimately, such MR arrays could be integrated on the same die with appropriate processing circuitry. For example, if the MR array were fabricated on a Si substrate then the processing circuitry would be also Si based. For higher operating temperatures, silicon-on-insulator (SOI) fabrication could be used. A potentially lower cost alternative to the SOI approach would be to take advantage of the fact that MRs are currently fabricated on GaAs, a high temperature semiconductor. In this regard, the integrated processing circuitry is fabricated on GaAs (or related InP) using HBT (Heterojunction Bipolar Transistor) or HEMT (High Electron Mobility Transistor) structures. This technology is now easily available and inexpensive through the explosive growth of the cellular phone industry.
Accordingly, what is needed is a compact and inexpensive die having at least two arrays of magnetosensitive elements and configured so as to produce an array geometry suitable for torque sensing schemes in which the output signals are capable of being directly coupled to digital signal processing electronics, such as a digital signal processor or microprocessor, whereby appropriate algorithms can provide, for example, target torque information, target position information, direction of target rotation, and target rotational speed information.
The present invention is a compact, inexpensive, high accuracy, non-contacting digital torque sensor employing a torsion bar, a target wheel at each end thereof, and at least two magnetosensitive (galvanomagnetic) sensors, in the form of magnetosensor device arrays (one for each target wheel, respectively), to precisely determine the angle of twist of the torsion bar (unlike present art compliant analog torque sensors employing a torsion bar and utilizing analog measuring techniques). The present invention also provides target wheel rotation speed, position and rotational direction information, is capable of self-compensation over wide temperature ranges and air gaps, including tilts, does not require tight assembly tolerances and has a theoretically infinite life due to its non-contact nature. The present invention can also be smaller than the present resistive film type torsion sensor and can meet anticipated future reduced size requirements.
The present invention employs two identical target wheels, having peripherally disposed magnetic irregularities preferably consisting of teeth and slots, fixedly attached to opposing ends of a torsion bar. By xe2x80x9cfixedly attachedxe2x80x9d is meant that both target wheels must rotate in unison with twist of the torsion bar at its respective attachment location. The teeth and slots of the target wheels do not have to be aligned in any particular way with respect to each other.
The teeth and slots of each target wheel are sensed by its respective magnetosensitive sensor, each being composed of an array of several magnetosensitive (galvanomagnetic) elements preferably manufactured on a single die to secure extremely accurate spacing between the elements. The arrays do not need to be placed at the same angular positions of the torsion bar or target wheels. The arrays may be located anywhere around the periphery of their respective target wheel provided that the array elements are aligned with the direction of target wheel rotation.
When the torque sensor is initially installed, each array is rapidly scanned by a microprocessor or a dedicated digital signal processor to compute the angular offset between the arrays corresponding to zero torque and zero differential displacement between the target wheels. This value is used in all subsequent torque computations and dispenses with any alignment requirements for either the target wheels or the arrays. Thereafter, when the torsion bar or target wheels are rotated or at a standstill, each array is rapidly scanned by a microprocessor or a dedicated digital signal processor to compute the precise torque induced phase shift between the target wheels, whereby appropriate algorithms can provide, for example, target torque information, target position information, direction of target rotation, and target rotational speed information. Scanning can be continuous or on demand only, depending upon the application, wherein the time required to scan both arrays is negligible compared to the speed of rotation of the target wheels (that is, relatively speaking, the target wheels are as if stationary during a scan).
Operatively, one target wheel is connected to a first component and the other target wheel is connected to a second component, wherein depending upon compliance of rotation between the first and second components, a torque therebetween arises which twists the torsion bar. Twisting of the torsion bar due to the torque results in a differential displacement between the magnetic irregularities of the target wheels as compared to a zero torque, zero differential displacement condition, wherein both wheels may be both rotating at any speed and for any number of revolutions. The maximum allowed differential displacement cannot exceed the length of the arrays, wherein preferably the length of the arrays exceeds the length of the pitch of the magnetic irregularities and the length of the pitch exceeds the differential displacement.
The arrays of magnetosensitive devices are incorporated, preferably, on at least one die in which the output signals are directly coupled to digital signal processing electronics, such as a digital signal processor or microprocessor, wherein the digital signal processing electronics may be, preferably, incorporated on the same die of magnetosensitive elements thereof, and whereby appropriate algorithms can provide, for example, target torque information, target position information, direction of target rotation information, and target rotational speed information.
According to a first aspect of the present invention, the torque sensor includes at least two arrays formed of a plurality of magnetosensitive elements, wherein the output signals are capable of being directly coupled to digital signal processing electronics, such as a digital signal processor or microprocessor, incorporated on the same die as that of the magnetosensitive elements. The arrangement of array elements is such as to provide a die suitable for use as a torque sensor, wherein signal processing is accomplished on a chip on the same die as that of the magnetosensitive elements such that appropriate algorithms can provide, for example, target torque information, target position information, direction of target rotation, and target rotational speed information.
According to a second aspect of the present invention, the torque sensor includes at least two arrays, each array on a separate die, whereby each array is formed of a plurality of magnetosensitive elements, wherein the output signals are capable of being directly coupled to a digital signal processor or microprocessor incorporated on one of the dies or elsewhere. The arrangement of the magnetosensitive elements is such as to be suitable for use as a torque sensor, wherein signal processing is accomplished such that appropriate algorithms can provide, for example, target torque information, target position information, direction of target rotation, and target rotational speed information.
According to the preferred embodiment of the first and second aspects of the present invention, magnetosensitive elements, preferably magnetoresistive (MR) elements, are incorporated in the arrays. The MR elements are arranged and configured so as to produce a variety of MR array geometries suitable for use as a torque sensor, wherein an MR array is defined as having three or more MR elements.
According to a preferred method of fabrication, an indium antimonide epitaxial film is formed, then masked and etched to thereby provide epitaxial mesas characterizing the MR elements. Shorting bars, preferably of gold, are thereupon deposited, wherein the epitaxial mesa not covered by the shorting bars provides MR segments of the MR element. The techniques for fabricating epitaxial mesas with shorting bars are elaborated in U.S. Pat. No. 5,153,557, issued Oct. 6, 1992, U.S. Pat. No. 5,184,106, issued Feb. 2, 1993, U.S. Pat. No. 5,491,461, issued Feb. 13, 1996, and U.S. Pat. No. 6,201,466, issued Mar. 13, 2001, each of which being hereby incorporated herein by reference.
Accordingly, it is an object of the present invention to provide a torque sensor incorporated on an MR die comprising at least one MR array according to the first and second aspects of the present invention.
This and additional objects, features and advantages of the present invention will become clearer from the following specification.